The present disclosure relates to a fixing device and an image forming apparatus including the same, and the present disclosure relates particularly to a fixing device configured to fix an unfixed toner image on a recording medium by inserting the recording medium through a fixing nip portion formed by a heating rotary body and a pressure roller, and an image forming apparatus including the same.
A fixing method widely used in conventional image forming apparatuses adopting the electrophotographic method is a heat roller fixing method in which a heating roller (heating rotary body) is formed by providing a heat source inside or outside a fixing roller, and a sheet (recording medium) carrying an unfixed toner image thereon is inserted through a fixing nip portion formed by the heating roller and a pressure roller while applying heat and pressure to the sheet, to thereby fix the toner image onto the sheet.
Besides, there has been developed a belt fixing method in which, instead of a heating roller, an endless-shaped fixing belt (heating rotary body), which is heated by a heat source, is used instead of a heating roller, and a sheet carrying an unfixed toner image thereon is inserted through a fixing nip portion formed by the fixing belt and a pressure member pressed against the fixing belt, to thereby fix the toner image onto the sheet. With the belt fixing method, in comparison with the heat roller fixing method, it is possible to reduce the thermal capacity to shorten warm-up time, and to reduce power consumption.
Known heating methods for heating such a heating roller and a fixing belt include a lamp method in which a lamp such as a halogen lamp is used to provide heating, but the recent demand for shorter warm-up time and energy saving, there has been proposed an induction-heating (IH) method in which an alternating magnetic field is interlinked with a magnetic conductor to cause an eddy current, and thereby heating is provided.
Here, in heating a heating rotary body (a fixing roller, a fixing belt), breakage of the heating rotary body, erroneous control operation, etc. may cause a sudden abnormal heating of the heating rotary body. In particular, a heating method with a small thermal capacity tends to suffer from such a sudden abnormal heating. When abnormal heating occurs in a region other than a region where a thermistor (temperature detecting portion) that detects a temperature of the heating rotary body is disposed, it takes a long time to detect the abnormal heating.
As a solution to this problem, there is known, for example, a fixing device in which a temperature-sensitive resistor made of a barium titanate semiconductor ceramic composition, of which the resistance varies according to temperature, is disposed to face an entire longitudinal region of a belt that generates heat, and abnormal heating of the belt is detected by detecting variations in resistance of the temperature-sensitive resistor.